Electromagnetic compatibility in the enclosure level of packaging is the ability of an electronic system or subsystem to readily operate in its intended electromagnetic environment regardless of responding to electrical noise or generating unwanted electrical noise. Electromagnetic interference (EMI) is the impairment of the performance of an electronic system or subsystem by an unwanted electromagnetic disturbance.
It is known that electromagnetic compatibility can be achieved by reducing the EMI below the level that disrupts the proper operation of the electronic system. This compatibility is generally accomplished by means of line filters and/or equipment shields.
In discussing electromagnetic compatibility and EMI, it can be useful to use the terms "emitter" and "susceptor", which refer, to a system or subsystem that generates noise and a system or subsystem that is susceptible to noise, respectively. Any barrier placed between an emitter and a susceptor that diminishes the strength of the interference can be thought of as an EMI shield. How well the shield attenuates an electromagnetic field is referred to as its shielding effectiveness, expressed in decibels (dB). Thus, shielding effectiveness is a measure of the ability of the shield to control radiated electromagnetic energy.
To achieve electromagnetic compatibility in electronics useful in both military and civilian applications, EMI shielding must be considered. Such shielding can be accomplished by incorporating proper choices of metallic components and packaging into the design and by proper selection of EMI gaskets, adhesives, sealants, or coatings. Useful EMI shielding materials are effective due to the presence of silver, silver-plated copper, silver-plated nickel, silver-plated glass, silver-plated inorganic material, nickel, copper, and/or other conductive filler. All of these materials are effective as EMI/RFI components due to their high conductivity. Silver-filled polymers with a lowest volume electrical resistivity (VER) of 0.0002 ohm-cm provide a shielding effectiveness of 60 dB for 0.0001 in. thickness. Reduction of the VER will improve the shielding effectiveness of the material.
Conductive polymers for die-bonding and other microelectronic applications are obtained by filling a sufficient amount of conductive particles into a polymer matrix which is normally insulating that the particles will be in close proximity with one another and thus facilitate current flow. The most conductive polymeric composition achieved to date by a manufacturer contains silver particles. The lowest volume electrical resistivity achieved is 0.0002 ohm-cm for an epoxy resin filled with from about 70 to 90% by weight of silver powder. Such a low resistivity is characteristic of any polymeric conductive material in which the polymeric material is not cured at a temperature above 200.degree. C. for an extended period of time, which curing tends to degrade the physical properties of the material.
Conductive compositions containing silver filler are disclosed in each of U.S. Pat. Nos. 4,407,674 and 4,518,524. The compositions of U.S. Pat. No. 4,407,674 which comprise silver powder of extreme thinness, have a relatively high VER and are thus not especially conductive. U.S. Pat. No. 4,518,524 is directed to electrically conductive silver coatings in which silver and ferro alloy are used as filler or pigment in binder resin.